Fix JSX comment duplication in preserve mode

[Copilot]

This PR fixes a bug where comments adjacent to JSX expressions were duplicated in the emitted output when using jsx: preserve mode.

Problem

When JSX contains comments immediately before or after expressions, TypeScript was emitting those comments twice:

// Input
const jsx = <div>/*pre*/{x}/*post*/</div>;

// Output (incorrect - comments duplicated)
const jsx = <div> /*pre*//*pre*/{x} /*post*//*post*/</div>;

Root Cause

The issue occurred because comments in JSX content are parsed as both:

1. JSX text nodes containing the comment text (e.g., JsxText with content "/*pre*/")
2. Trivia comments attached to adjacent JSX expressions

This caused the same comment content to be emitted twice:

• Once when emitting the JSX text node via emitJsxText()
• Again when emitting trivia comments via the comment emission logic in emitTokenWithComment()

Solution

The fix prevents duplicate comment emission by:

1. Adding context tracking: Introduced isEmittingJsxChildren flag to track when we're emitting JSX element children
2. Suppressing duplicate trivia: Modified emitTokenWithComment() to skip leading/trailing comment emission for JSX expressions when they would duplicate JSX text content
3. Preserving correct behavior: The fix only affects JSX expressions within JSX children context, leaving other comment emission unchanged

Testing

Added comprehensive test case covering:

• Basic case: <div>/*pre*/{x}/*post*/</div>
• Original issue: <App>/* no */{/* 1 */ 123 /* 2 */}/* no */</App>
• Edge cases: Leading-only and trailing-only comments

The fix works for both jsx: preserve and jsx: react-jsxdev modes.

Addressing #11.

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